The opportunity to attenuate some of these hazards is not limited to the munitions only but has expanded to include their containers as well. The technological advancements in this sector include impact fortification, thermally protective coatings, and thermally-activated panel venting using thermoplastic materials. The next generation of insensitive munitions container technology is emerging to improve performance and provide better reliability to ensure proper venting of munition containers during thermal events without compromising the containers' bullet/fragment impact resistance.
Under normal operating conditions, most containment devices are both effective and relatively safe. These containment devices provide an essential function in storing items and preventing the unwanted spread of the various materials stored in them. These containment devices are especially useful in preventing environmental contamination of explosive or hazardous material. In particular these containment devices are used in modern munitions. In munitions, containment devices provide an essential military capability and are designed to vent allowing the contents to burn, thus preventing an explosion or violent rupture from a build up of pressure.
However, when munitions, such as solid rocket motors (SRMs), are placed under unwanted stimuli, like heat and mechanical shock, dangerous results can occur. Munitions may be triggered to ignition by fire or by impact with bullets or fragments. As a result there is a push to develop a class of munitions that are insensitive munitions (IM). An IM is one that will not detonate under any conditions other than its intended mission. If an IM is struck by fragments from an explosion or hit by a bullet, it should not detonate or explode. Additionally, an IM should not detonate if it is in close proximity to a target that is detonated. In extreme temperatures, insensitive munitions will only burn (without detonation or explosion). The fact that IM will not explode if in close proximity to a detonation and will burn without explosion, allows greater numbers of IM to be packaged, handled, stored, and transported in smaller containers because more items can be stored in these smaller areas and they require shorter distances between storage facilities. However, due to this higher concentration of IM storage, the requirements of IM containers demand higher levels of safety performance. It also allows for cost saving opportunities because more munitions will be able to be stored safely in a smaller area.
As a result of a number of well publicized accidents in recent years involving premature and inadvertent activation of munitions with resultant loss of life among service personnel as well as other damage, there has been an increased emphasis on designing munitions that are safer to store, handle and use. Specifically, in October 2006, in Baghdad, Iraq, a fire erupted in a U.S. ammo dump, setting off a series of blasts due to the detonation of the ammunition that was stored in the facility. This is a prime example of the need for current and future munitions to be IM compliant.
As munition systems are developed or upgraded, they will be required to meet a growing set of specifications for IM compliance. The current technologies that are used to minimize the effects of unintentional stimuli on munitions are insufficient and new IM technologies must be developed to allow IM compliance in future systems. One of the first lines of defense in protection of munitions is the containers that they are stored in during inactive periods. These containers can protect the munitions from bullets and fragments and can serve as a containment system in slow and fast cook-off situations. However, during slow and fast cook-offs of the munition, the container must be capable of sufficiently venting the generated gasses. Current containers are sealed to protect the munition from the outside environment; however, to reach IM compliance, containers must be capable of venting when a set of predetermined conditions are met. SRM cases can also be vented to allow for IM-compliant systems.
In addition to the venting needs of munition containers, venting systems are necessary for various types of containers, such as pressure vessels, grain silos, plumbing systems, fire suppression systems, buildings, and holding tanks. Typically safety panels are incorporated into the roofs or walls of these structures to relieve pressure in the event of an explosion or other sources of high pressure build up in order to relieve pressure to prevent a collapse. However, many of the existing products are based solely on a pressure increase as a stimulus and are not reusable or capable of undergoing non-destructive performance verification testing.
Based on recent incidents with the unintentional detonation of munitions, there has been increased research into the area of creating better, more efficient IM and IM containers. Most of these newer systems rely on passively activated designs to vent their explosive material to the environment before environmental conditions can activate the propellants or explosives.
Current IM container systems, such as those manufactured by Conco, Inc., utilize panels or plugs that melt under thermal stimuli in order to release containment. The panels could be comprised of plastic, rubber, fiberglass, or any other material whose properties allow for containment under normal operating conditions but degrade when exposed to thermal stimuli. However, these panels or plugs can compromise the bullet/fragment impact protection of the vessel in those regions. Additionally, in the event that the melt panel is exposed to a thermal stimulus, the container must be returned to the manufacturer for refurbishment.
Several systems have been developed that use shaped explosive products to create holes in the munitions or propellant containers in order to release containment. However, the explosive products or the impact of a bullet, fragment, or shaped charge jet from typical storage systems may result in a temperature sufficient to ignite the propellants or munitions.
One system utilizing a charge is described in U.S. patent application Ser. No. 11/261,184 filed Oct. 28, 2005 by Skinner. Skinner discloses a device for venting a container housing and energetic material including an installation portion, a charge holder disposed in the insulating portion, and an explosive cutting charge disposed in the charge holder. The device further includes a thermally activated initiation device, and a transfer line coupling the thermally activated initiation device and the explosive cutting charge. When exposed to a temperature at or above a predetermined temperature heat produced deflagration of the thermally activated initiation device initiates deflagration in the transfer line which in turn detonates explosive cutting charges. Upon detonation these explosive cutting charges perforate the container to relieve pressure or avoid buildup of pressure within the case.
In U.S. Pat. No. 6,338,242 issued to Kim et. al on Jan. 15, 2002 a passive ordnance venting system is disclosed. In Kim, the system has an ordnance device, a casing with a vent opening, a dome plug fitted into the formed vent opening and an adapter fitted over the dome plug on the outside of the casing. The adapter connects sufficiently to the casing to retain the dome plug against the formed vent opening for given pressures. The adapter melts at high temperatures and releases the dome plug to reduce the danger of explosion from heat induced over pressurization.
Fuses, charges or plugs do allow venting however; many may be required to achieve adequate venting. Additionally, these venting methods lack durability, and do not increase the function of containers, such as ease of use or access.
In addition, venting systems are necessary for various types of containers, such as pressure vessels, grain silos, and buildings. It is typical to incorporate safety panels into the roofs or walls of buildings such as laboratories, testing facilities, and manufacturing plants in order to relieve pressure in the event of explosions or other sources of high pressure build-up. This is necessary to prevent the structure from collapsing and to minimize the injury to persons inside the structure. A specific application would be the prevention of pressure build up in nuclear reactor containment buildings. Existing blow-out panels, however, are difficult to adjust accurately to the pressure at which a particular panel will blow out.
Commercially available venting systems for building structures are available through suppliers such as Construction Specialties, Inc. and Oseco, Ltd. These systems rely on panels that rupture or hinged panels that require complex calibrated release mechanisms. As such, this added complexity can increase reliability issues. Furthermore, especially with a rupture panel, these existing systems cannot be tested for functionality. Finally, explosion vents are designed to be the weakest part of the external structure, thus can significantly impact the structural integrity of the parent structure. Additionally, they can pose a potential security risk in that intruders can easily access the structure through a rupture type panel.
Shape memory materials were first developed about twenty years ago and have been the subject of commercial development in the last ten years. Shape memory materials derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. There are principally two types of shape memory materials, shape memory alloys (SMAs) and shape memory polymers (SMPs).
SMAs and SMPs that have been preformed can be more easily deformed to a desired shape above their glass transition temperature (Tg). The SMA and SMP must remain below, or be quenched to below, the Tg while maintained in the desired shape to “lock” in the deformation. Once the deformation is locked in, the SMA, because of its crystalline network, and the SMP, because of its polymer network, cannot return to a relaxed state due to thermal barriers. The SMA and SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereupon the SMA and SMP stored mechanical strain is released and the SMA and SMP returns to its preformed state.
There are principally two types of plastics, thermoset resins and thermoplastic resins, each with its own set of unique characteristics. Thermoset resins, for example polyesters, are liquids that react with a catalyst to form a solid, and cannot be returned to their liquid state, and therefore, cannot be reshaped without destroying the polymer networks. Thermoplastic resins, for example PVC, are also liquids that become solids. But unlike thermoset resins, thermoplastics are softened by application of heat or other catalysts. Thermoplastics can be heated, reshaped, heated, and reshaped repeatedly.
SMPs used in the presently disclosed device are unique thermosetting polymers that, unlike traditional thermosetting polymers, can be reshaped and formed to a great extent because of their shape memory nature and will not return to a liquid upon application of heat. Thus by creating a shape memory polymer that is also a thermosetting polymer, designers can utilize the beneficial properties of both thermosetting and thermoplastic resins while eliminating or reducing the unwanted properties. Such polymers are described in U.S. Pat. No. 6,759,481 issued to Tong, on Jul. 6, 2004 which is incorporated herein by reference. Other such thermoset resins are seen in PCT Application No. PCT/US2006/062179, filed by Tong, et al on Dec. 15, 2006; and PCT Application No. PCT/US2005/015685 filed by Tong et al, on May 5, 2005 of which both applications are incorporated herein by reference.
There are three types of SMPs: 1) A partially cured resin, 2) thermoplastics, and 3) fully cured thermoset systems. There are limitations and drawbacks to the first two types of SMP. Partially cured resins continue to cure during operation and change properties with every cycle. Thermoplastic SMP “creeps,” which means it gradually “forgets” its memory shape over time. A thorough understanding of the chemical mechanisms involved will allow those of skill in the art to tailor the formulations of SMP to meet specific needs, although generally fully cured thermoset resin systems are preferred in manufacturing.
While SMA and SMP appear to operate similarly on the macro scale, at the molecular scale it is apparent that the method of operation of each is very different. The difference between SMA and SMP at the molecular level is in the linkages between molecules. SMA essentially has fixed length linkages that exist at alternating angles establishing in a zigzag patterned molecular structure. Reshaping is achieved by straightening the angled connections from alternating angles to straight forming a cubic like structure. This method of reshaping SMA material enables bending while limiting any local strains within the SMA materials to less than 8% strain, as the maximum shape memory strain for SMA is 8%. This 8% strain allows for the expansion or contraction of the SMA by only 8%, a strain that is not useful for most industrial applications. Recovery to memory shape is achieved by heating the material above a certain temperature at which point the molecules return to their original zigzag molecular configuration with significant force thereby reestablishing the memory shape. The molecular change in SMA is considered a metallic phase change from Austenite to Martensite which is defined by the two different molecular structures.
SMP has connections between molecules with some slack. When heated these links between connections are easily contorted, stretched and reoriented due to their elastic nature as the SMP behaves like an elastic material when heated; when cooled, the shape is fixed to how it was being held. In the cooled state the material behaves as a typical rigid polymer that was manufactured in that shape. Once heated the material again returns to the elastic state and can be reformed or return to the memory shape with very low force. Unlike SMA which possess two different molecular structures, SMP is either a soft elastomer when heated or a rigid polymer when cooled. Both SMA and SMP can be formulated to adjust the activation temperature for various applications. Critical to the success of the currently claimed device is thermoset SMP which provides an order of magnitude higher stiffness than previous state-of-the-art thermoplastic SMPs.
Shape memory alloys have been used to attempt to solve the IM issue as disclosed in U.S. Pat. No. 6,321,656 issued to Johnson on Nov. 27, 2001. In Johnson a thermally responsive material, such as Nitinol, an SMA, is used to create a latching mechanism which, upon exposure to temperatures in excess of the Tg of the Nitinol changes shape so as to mechanically unlatch the rocket casing section holding the propellant. However, as noted above the shape change is minimal, requiring very large mechanisms to ensure unlatching or thinner SMA as the latching mechanism which may fail due to a lack of structural strength. Additionally, Johnson does not describe or disclose the use of an SMP or SMP composite for use in a venting system for IM.
The term “composite” is commonly used in industry to identify components produced by impregnating a fibrous material with a thermoplastic or thermosetting resin to form laminates or layers. Generally, polymers and polymer composites have the advantages of weight savings, high specific mechanical properties, and good corrosion resistance, which make them indispensable materials in all areas of manufacturing. Because SMPs are resins, they can be used to make composites, which are referred to in this application as SMP composites.
Unlike SMAs, SMPs exhibit a radical change from a normal rigid polymer to a flexible elastic and back on command. SMAs would also have issues with galvanic reactions with other metals which would lead to long term instability. The current supply chain for SMAs is currently not consistent as well. SMP materials offer the stability and availability of a plastic and are more inert than SMAs. Additionally, when made into a composite SMPs offer similar if not identical mechanical properties to that of traditional metals and SMAs in particular. Throughout this disclosure SMP and SMP composites are used interchangeably as each can be replaced by the other depending on the specific design requirements to be met.
Therefore there is a need for a reliable and reusable venting system that can meet the requirements needed to transport and store munitions, vent the munition containers in high temperature environments (cook-off scenarios), yet still maintain bullet/fragment impact resistance requirements. In addition, there is also a need for a more functional venting system for building type structures and other containers.